Aromatic polyester resins, as represented by polyethylene terephthalate, are excellent in shapability, mechanical properties, transparency, etc. and are widely used as a packaging material for various foods and containers for beverages, etc. However, as a packaging material, particularly for foods to be stored for a long period, the gas-barrier property of an aromatic polyester resin is not sufficient so that the deterioration of contents has been inevitable.
On the other hand, polyglycolic acid resin is known to have particularly excellent gas-barrier property in addition to heat resistance and mechanical strength (e.g., Patent document 1 listed below), and it has been proposed to add a small amount thereof to an aromatic polyester resin to provide an aromatic polyester resin composition improved in gas-barrier property of the latter (Patent documents 2 and 3). However, aromatic polyester resin and polyglycolic acid resin are basically non-compatible with each other, and the resin composition obtained by melt-kneading these resins is caused to have a poor transparency. Moreover, it is thought that the improvement in gas barrier property by addition of a relatively small amount of polyglycolic acid resin into aromatic polyester resin is based on the gas interception effect of the polyglycolic acid resin disperse phase dispersed in the matrix of aromatic polyester resin (Nonpatent document 1 listed below). According to the thought, it is understood that the lowering in transparency of such a blend composition is essentially inevitable. On the other hand, Patent document 3 has proposed to add a coupling agent, such as anhydrous pyromellitic acid, at the time of melt-kneading of an aromatic polyester resin and a relatively small amount of polyglycolic acid resin, or to use a copolymer of polyglycolic acid and isophthalic acid, etc., instead of polyglycolic acid, in order to suppress the lowering of transparency. However, in this case, while the lowering of transparency can be suppressed, the effect of improving the gas barrier property becomes scarce.    Patent document 1: JP-A 10-60136    Patent document 2: U.S. Pat. No. 4,565,851    Patent document 3: JP-A 2005-200516    Nonpatent document 1: L. E. Nielsen; J. Macromol. Sci. (CHEM), A1 (5), 929-942 (1967).